Most engineers active in the offshore energy business have, at some point in their career, sat in on a HAZID meeting where a critical risk had to be mitigated. One of the attendees (typically a manager) suggests that a monitoring campaign should be carried out. A wonderful idea!
Most stakeholders tend to agree, because they see only benefits. Monitoring does not affect operations, costs are limited as compared to typical day rates (let alone the consequences of the risk it intends to mitigate) and everybody looks like they are taking responsibility.
A monitoring campaign is devised to measure and check an offshore operation, for instance a complex heavy lift. It could include the following:
- Motion Reference Units (MRU) for measuring quickly varying motions in six degrees of freedom of the offshore unit and the load. MRUs measure accelerations in translation and velocities in rotation. These can be converted to position by integration over time, minding drift. Since gravity is an acceleration, MRUs know where is up and where is down. Every smartphone has an MRU and to many it is somewhat preposterous that not every object offshore is equipped with one.
- High-precision GPS for measuring longer term motions in the horizontal plane (position and orientation). These systems are typically used for positioning of mobile offshore units. They are relatively expensive and are continuously operated by a specialist and his back-to-back.
- Wave rider buoy or downward looking wave radar. These frequently used systems measure the instantaneous water surface level at great accuracy at a specific location. Processing to identify the wave energy density distribution over distinct frequencies is commonplace, but over distinct directions is hard and often impossible.
- Wind sensor / anemometer. Simple and cheap measurement device for wind speed at a specific location. Measurement quality is highly dependent upon location and vulnerable to shielding effects.
- Current measurement device (either propeller or acoustic based). These devices measure current at a specific location either at a specific depth or for the entire water column. When hung of from a vessel, the vessel presence will severely influence the measurement.
- Strain gauges. Contrary to popular belief, these devices do not measure stress (let alone force) but accurately measure local strains. Many strain gauges, at often hard to reach places, in addition to detailed knowledge and assumptions about a structure are required to translate strain measurements to cross-sectional stress and force.
One often overlooked form of measurement is a calibrated environmental now-cast. Now-cast is similar to environmental forecast and provides detailed wind, wave and current information. The now-cast is based on a detailed model of the sea and weather and is continuously calibrated based upon real-time (mostly satellite-based) measurement. The now-cast, more than any measurement, aligns greatly with the desktop engineering reality as well as the on-board engineering reality due to the similarity with the environmental forecast. Much more about this in an upcoming article.
Now, back to how most offshore monitoring campaigns come into existence.
Engineers enjoy performing monitoring campaigns because something is to be learned about the on-board reality, which is greatly different from the desktop reality. HSE officers like them because they provide a sense of control over risks. Managers like them because they provide big data and a sense of ownership. Offshore unit owners like them, because they feel that somehow the acquired data can be capitalized upon for future projects.
And thus, during already stressful mobilization, an uncoordinated pack of measurement specialists boards the offshore unit and starts planting devices, pulling wires and fighting with the chief electrician over whether the right ports are opened on the firewall or not.
The offshore operations commence, and measurement starts interfering (‘Can we perform this lift if we don’t receive signal from our wave rider buoy?’, ‘The measurements and forecast disagree, should we use the most onerous?’, or, more existentially: ‘If my current position cannot be accurately measured, am I really here?’).
Usable measurement results are expected once the operation is done and over. Reports are provided from the various parties performing measurements. Desk studies trying to tie all the data together into firm, usable conclusions tend to struggle because some crucial element is unclear (‘Why is there a sudden list occurring here?’, ‘Why do the motion measurements seem to conflict?’, ‘The MRU knows where is up, by why did no one report whether the x-axis aligns with forward or starboard?’). Engineers may be pressed to declare that, based on the measurements, the operation was even more safe than anticipated.
None of the expensively acquired data can be used to capitalize upon for a next similar operation, because without a firm understanding of underlying physics, minor differences cannot be accounted for. The operation was successful, so everyone will be happy at the end, but the success of the monitoring campaign was nothing more than a very expensive ticking the box.
What went wrong? Are monitoring campaigns worthless exercises? What can be done to improve this? Did it occur to you, reader, that at the start of this article, the contents of a monitoring campaign were introduced as a mere list of measurement devices?
Measurements alone will lead to data only. A measurement campaign should start with questioning which engineering uncertainties are to be minimized through measurement. The answer to that question allows an experienced engineer, with a strong basis in both desktop reality and on-board reality, to devise a scope of work for analysis to be performed. One of the inputs into the analysis is well-specified measurement data. The analysis should lead to a firm grasp on the physics behind the actual behavior of the offshore spread. The monitoring campaign should bridge the gap between the on-board engineering reality and the desktop engineering reality.
This first article introduced the philosophy of ‘gap-bridging monitoring’. In my next article I will present this philosophy in more detail and apply it to a typical jack-up performing a recurring operation.